The invention relates to chromogenic oxazines useful for detection of cyanide.
The cyanide anion is a particularly strong nucleophile and forms stable complexes with a variety of transition metals in aqueous solution.1-3 In fact, a wealth of diverse industrial applications have been developed around the excellent binding properties of this particular ligand.4-5 The strong affinity of cyanide for transition metals, however, has deleterious consequences on cell metabolism.6-8 Specifically, this anion binds the active site of cytochrome oxidase and inhibits the mitochondrial electron-transport chain. As a result, cyanide is extremely toxic and even relatively small amounts (e.g., 0.5-3.5 mg per kg of body weight) are lethal to humans.9 Unfortunately, cyanide does not easily decompose in the environment.10 Therefore, the accidental spillage of this toxic chemical from industrial plants, or even its intentional release, can contaminate drinking waters and become a serious threat to human health. Indeed, the concentration of cyanide in drinking water cannot be greater than 1.9 μM in accordance with the World Health Organization standard.11 
Numerous standard methods for the detection of micromolar amounts cyanide in water have been developed relying on a diversity of experimental protocols and detection techniques.12 Most of such strategies, however, require either multistep procedures with tedious sample pretreatments or sophisticated instrumentation. The development of chemosensors13-18 for the recognition of anions17-31 can facilitate the qualitative, and perhaps even the quantitative, determination of cyanide. In particular, the identification of chromogenic compounds that respond to the presence of cyanide anions with fast and visible color changes would offer the opportunity to screen rapidly water samples relying exclusively on the naked eye. Indeed, a few organic molecules and transition metal complexes able to signal the presence of cyanide with pronounced changes in their absorption and emission properties have been identified.32-37 Their operating principles are based on hydrogen bonding interactions, metal coordination, or the formation of covalent bonds between the nucleophilic cyanide anion and compatible electrophilic centers. Some of the chemosensors can even detect micromolar amounts of cyanide.36-37 But most of them suffer from the deleterious interference of other anions.32-37 Halide anions in particular, and especially fluoride, tend to mask the response of cyanide of such chemosensors.32,34b,35,37c 
The present invention is directed to improved detection of cyanide that addresses the aforementioned problems by designing heterocyclic compounds for the calorimetric detection of cyanide. The skeleton of our molecules fuses a benzooxazine ring to an indoline fragment and can be efficiently synthesized from readily available precursors. In the presence of cyanide, our molecules are converted into cyanoamines with the concomitant appearance of an intense band in the visible region of the absorption spectrum. The developing absorption is a result of the opening of the benzooxazine ring with the formation of a phenolate chromophore. Nuclear magnetic resonance spectroscopy and X-ray crystallographic analyses demonstrate that the covalent attachment of a cyanide anion to the indoline fragment is responsible for this transformation. The chromogenic process is particularly fast for the methyl-substituted oxazine and can be exploited to detect micromolar concentrations of cyanide in water. Furthermore, the calorimetric response of our compounds to cyanide does not suffer the interference of the halide anions, which instead are known to complicate the detection of cyanide in conventional sensing protocols. Thus, our mechanism and compounds for the colorimetric identification of cyanide can lead to the development of practical strategies for the convenient determination of this toxic anion in aqueous environments.
Compounds, compositions, articles (e.g., test kits or single-use devices), and processes for using and making the aforementioned products are provided. Other advantages and improvements are described below or would be apparent from the disclosure herein.